Fix compilation warning.

8 jobs for 4.5.0-rc in 26 minutes and 25 seconds (queued for 22 minutes and 16 seconds)
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Name Stage Failure
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format Test
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robotpkg-hpp-pinocchio-18.04-debug Test
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robotpkg-hpp-pinocchio-14.04-debug Test
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robotpkg-hpp-pinocchio-14.04-release Test
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